[iOS] Tweak tab grid page control background.

Per design recommendation, this CL adjusts the color and alpha of the
tab grid page control.

Bug: 849646
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ia2fe6c4709994f8c763e76c1960afe096305b460
Reviewed-on: https://chromium-review.googlesource.com/1122117Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572611}

ios/chrome/browser/ui/tab_grid/tab_grid_page_control.mm

@@ -97,9 +97,9 @@ const NSTimeInterval kSliderMoveDuration = 0.2;
 // Color for the slider
 const int kSliderColor = 0xF8F9FA;
 // Color for the background view.
-const int kBackgroundColor = 0x5F6368;
+const int kBackgroundColor = 0xFFFFFF;
 // Alpha for the background view.
-const CGFloat kBackgroundAlpha = 1.0;
+const CGFloat kBackgroundAlpha = 0.3;
 // Color for the regular tab count label and icons.
 const CGFloat kSelectedColor = 0x3C4043;